1. Field of the Invention
This invention relates generally to a GaN semiconductor device and, more particularly, to a GaN high electron mobility transistor (HEMT) device fabricated on a silicon carbide (SiC) substrate, where the substrate includes a back-side via that includes diamond.
2. Discussion of the Related Art
Integrated circuits are typically fabricated by epitaxial fabrication processes that deposit or grow various semiconductor layers on a substrate to provide the circuit components for the device. Substrates for integrated circuits can include various materials, usually semiconductor materials, such as silicon, sapphire, SiC, InP, GaAs, etc. As integrated circuit fabrication techniques advance and become more complex, more circuit components are able to be fabricated on the substrate within the same area and be more closely spaced together. Further, these integrated circuit fabrication techniques allow the operating frequencies of the circuit to increase to very high frequencies, well into the GHz range.
HEMT devices are popular semiconductor devices that have many applications, especially high frequency and high power applications, for example, power amplifiers. GaN HEMT devices are typically epitaxial grown on a suitable substrate for these applications, where the substrate needs to be highly thermally conductive, electrically insulative, have a thermal expansion coefficient similar to GaN and provide lattice spacing matching for suitable epitaxial growth. Suitable materials that are both highly thermally conductive and electrically insulative are relatively unique.
A high thermally conductive substrate is necessary so that heat is removed from the device junction through the epitaxial layers and the substrate so that the device is able to operate at high power in a reliable manner. Particularly, as the temperature of the device increases above some threshold temperature, the electrical performance of the device is reduced, which reduces its high power capability. Further, too high of a temperature within the device reduces its reliability because its time to failure will be reduced. Also, these types of devices are typically high frequency devices, which become smaller in size as the frequency increases, which reduces their ability to withdraw heat. The conductive path for heat generated at the device junction layer in an HEMT device causes the heat to propagate through the epitaxial layers and the substrate and into the device packaging. Therefore, it necessary to provide a high thermally conductive substrate that does not impede the path of the heat exiting the device, and allows the heat to spread out over a larger area. The thickness of the substrate is optimized to provide a low resistance heat path into the packaging from the device and provide the ability to spread the heat out away from the device.
For GaN HEMT devices, silicon carbide (SiC) substrates are currently the industry standard for providing the desirable characteristics of electrically insulating, highly thermally conductive, a close lattice match to that of GaN and a similar thermal expansion coefficient to that of GaN. However, although SiC is a good thermal conductor, its thermal conductivity is still limited, and as the junction temperature rises in the device, the ability of the SiC substrate to remove the heat is limited, which limits the output power of GaN HEMT devices, and subsequently their reliability, as discussed above.
It is desirable to provide a suitable substrate for a GaN HEMT device that has a greater thermal conductivity than SiC. Diamond is electrically insulating and has the highest thermal conductivity of any bulk material. However, it is currently not possible to epitaxial grow GaN layers on large area single-crystal diamond substrates for many reasons, including availability, a large lattice spacing mismatch and different thermal expansion coefficients. Efforts have been made in the industry to overcome these problems so as to use diamond substrates in a semiconductor device, such as GaN HEMT devices. For example, it is known in the art to remove the SiC substrate, or other substrate, that the GaN layers can effectively be grown on, and then bond a diamond substrate to the device using a bonding layer. However, there is now a bonding layer of some thickness between the GaN device layers and the diamond substrate that does not have the proper thermal conductivity, and thus affects the ability of heat to be removed from the device through the diamond substrate. Further, because bulk diamond has a low thermal coefficient of expansion, there is still the problem that the difference between the thermal expansion coefficients of the device layers and the substrate causes wafer curvature and possibly epitaxial layer cracking.
It is also known in the art to grow diamond on the front-side of the device opposite to the substrate. However, it has been shown that these types of devices have limited improvement in thermal conductivity and heat flow out of the device because heat flow through the substrate is still highly important. Further, GaN layers may not survive the high temperature diamond deposition process, and thus may need to be protected using a thermally resistive layer, which again limits the thermal performance.